I drew up this table about sp farming.

Basically as your gap increases, you gain less and less % of sp and loss more and more % of exp. So the worst way to sp farm is to no gap follow by a big gap. The best way is to keep a constant 2-3 level gap all the way.
Even if you do not want to sp farm, you should keep AT LEAST a 1 level gap, since that is the only time that your return of sp exactly equals your lost of exp.